Fast fluidized bed reactor and method of operating the reactor

ABSTRACT

A fast fluidized bed reactor, comprising an upright reaction chamber for containing a bed of granular material, the chamber having a cylindrical upper region and a lower region; a feeder for feeding matter into the lower region; apparatus for supplying pressurized air to the reaction chamber to fluidize the granular material in the circulating regime, whereby a portion of the granular material is entrained into the upper region; apparatus for tangentially supplying pressurized air to the upper region, the second stream of air being supplied, and the reactor being constructed, in a manner to provide a Swirl number of at least 0.6 and a Reynolds number of at least 18,000 in the upper region, thereby creating a cyclone of turbulence; apparatus for separating the granular material and the reaction gases exiting from the reaction chamber and returning the separated granular material to the lower region; a second fluidized bed fluidized in the bubbling regime and situated in the lower region adjacent a partition formed in the lower region for partitioning the circulating fluidized bed from the second fluidized bed, the second fluidized bed fluidizing the granular material separated from the reaction gases and the partition being situated to permit a portion of the granular material from the second fluidized bed to overflow into the circulating bed; and a heat exchanger immersed in the second fluidized bed.

This is a division of application Ser. No. 331,767, filed Dec. 17, 1981,U.S. Pat. No. 4,469,050.

BACKGROUND OF THE INVENTION

The present invention relates to an improved circulating, i.e., fast,fluidized bed reactor utilizing a cyclone of turbulent gases in theupper region of the reaction chamber, and to a method of operating thereactor; and, more particularly, to a reactor of this type utilizingcyclone particle separators and to a reactor of this type wherein suchcyclone separators are eliminated.

The present invention has specific application, inter alia, to adiabaticfluidized bed combustors, fluidized bed boilers, and fluidized bedgasifiers. As used herein, and in the accompanying claims, "adiabaticcombustor" denotes a fluidized bed combustor that does not containinternal cooling means, and "boiler" denotes a fluidized bed combustorthat contains internal heat absorption means, in the form of immersedboiler, superheater, evaporator, and/or economizer heat exchangesurfaces. The temperature of adiabatic fluidized bed combustors istypically controlled by the use of pressurized air in substantial excessof the stoichiometric amount needed for combustion. On the other hand,fluidized bed boilers require very low excess air, so that heatabsorption means are required in the fluidized bed. Fluidized bedgasifiers, in contrast, utilize less than stoichiometric amounts of air.

The state of fluidization in a fluidized bed of solid particles isprimarily dependent upon the diameter of the particles and thefluidizing gas velocity. At relatively low fluidizing gas velocitiesexceeding the minimum fluidizing velocity, e.g., a fluidization numberin the range from about 2 to 10, the bed of particles is in what hasbeen termed the "bubbling" regime. Historically, the term "fluidizedbed" has denoted operation in the bubbling regime. This fluidizationmode is generally characterized by a relatively dense bed having anessentially distinct upper bed surface, with little entrainment, orcarry-over, of the bed particles (solids) in the flue gas, so thatrecycling the solids is generally unnecessary. At higher fluidizing gasvelocities, above those of the bubbling regime, the upper surface of thebed becomes progressively diffuse and carry-over of the solidsincreases, so that recirculation of solids using a particulateseparator, e.g., a cyclone separator, becomes necessary in order topreserve a constant solids inventory in the bed.

The amount of solids carry-over depends upon the fluidizing gas velocityand the distance above the bed at which the carry-over occurs. If thisdistance is above the transfer disengaging height, carry-over ismaintained at a constant level, as if the fluidizing gas were"saturated" with solids.

If the fluidizing gas velocity is increased above that of the bubblingregime, the bed then enters what has been termed the "turbulent" regime,and finally, the "fast," i.e., "circulating," regime. If a given solidsinventory is maintained in the bed, and the fluidizing gas velocity isincreased just above that of the turbulent regime, the bed density dropssharply over a narrow velocity range. Obviously, if a constant solidsinventory is to be preserved in the bed, the recirculation, or return,of solids must equal the carry-over at "saturation."

At fluidizing gas velocities below those associated with theaforementioned sharp drop in bed density, the effect upon bed density ofreturning solids to the fluidized bed at a rate well above the"saturation" carry-over is not marked. The addition of solids to a bedfluidized in either the bubbling or turbulent regime at a rate above thesaturation carry-over will simply cause the vessel containing thefluidized bed to fill up continually, while the fluidized density willremain substantially constant. However, at the higher fluidizing gasvelocities associated with the fast regime, the fluidized densitybecomes a marked function of the solids recirculation rate.

Fast fluidized beds afford intimate contact between the high velocityfluidizing gas and a large inventory of solids surface per unit bedvolume. Also, slip velocity (i.e., solids-fluidizing gas relativevelocity) is relatively high in fast fluidized beds, when compared withthat in ordinary fluidized beds. Additionally, the combustion processwhich takes place in a fast fluidized bed combustor is generally moreintense, having a higher combustion rate, than that occurring intraditional fluidized bed combustors. Furthermore, as a result of thehigh solids recirculation rate in fast fluidized beds, the temperatureis essentially uniform over the entire height of such combustors.

The higher combustion reaction rate, compared to that of ordinaryfluidized bed combustors, allows the combustion temperature in fastfluidized bed combustors to be significantly reduced. Reduction of thecombustion temperature may be accomplished, for example, by insertingheat exchanger tubes in the combustion region. Reducing the combustiontemperature leads directly to a reduction in the total cost ofconstructing fast fluidized bed boilers, since (1) the total boiler heatexchange surface can be reduced, (2) thinner refractory bed liners arerequired, and (3) smaller cyclone separators can be installed. Moreover,contrary to prior art teachings, wet biomass materials may be combustedat such reduced combustion temperatures.

Notwithstanding the many advantages offered by fast fluidized bedreactors, as enumerated above, the high cost of constructing andmaintaining the extremely large external separation cyclones and largediameter standpipe required for recirculation of the entrained solids atthe rate necessary to maintain the bed in the fast fluidization regimeconstitutes a severe economic impediment to widespread commercialutilization of such reactors. In this regard, prior art fast fluidizedbed combustors are known which employ heat exchanger tube-lined walls inthe entrainment region of the combustor (i.e., parallel to the flow).Such combustors rely primarily on the transfer of radiant heat fromgases which typically are heavily laden with solids. Nevertheless, suchcombustors require an extremely large internal volume. Furthermore,still higher combustion rates are desired in fast fluidized bed boilers,with a concomitant reduction of the combustion temperature, and thus thesize of the combustor so as to reduce the cost of construction.

In the past, cyclone combustors which produce a cyclone of turbulentgases within the combustion chamber have been employed for combustingvarious solid materials, including poor quality coal and vegetablerefuse, as disclosed, for example, in "Combustion in Swirling Flows: AReview," N. Syred and J. M. Beer, Combustion and Flame, Vol. 23, pp.143-201 (1974). Such cyclone combustors do not, however, involve the useof fluidized beds.

Although providing high specific heat release, prior art cyclonecombustors suffer the following disadvantages: (1) the size of theusable fuel particles is limited to 0.25 inch (average effectivediameter); (2) fuel moisture content is limited to about 3-5%; (3) atclose to stoichiometric combustion, there is no means to controlcombustion temperature below the fusion point; and (4) erosion ofrefractory linings may occur in some instances.

Although the ordinary fluidized bed incinerator system described in U.S.Pat. No. 4,075,953 to Sowards, for example, is provided with a vortexgenerator, this system does not exhibit the combustion characteristicsassociated with conventional prior art cyclone combustors. Inparticular, the specific heat release is quite low (about 0.2×10⁶ 6 Kcalper cubic meter per hour) and the Swirl number [defined in terms ofcombustor input and exit parameters as S=(Input Axial Flux of AngularMomentum)/(De/2× Exit Axial Flux of Linear Momentum), where D_(e) is thecombustor exit throat diameter] is no greater than about 0.07.

Likewise, while the conventional combustion furnace described in U.S.Pat. No. 4,159,000 employs tangentially disposed air inlets, it does notachieve the combustion characteristics of conventional cyclonecombustors (e.g., it exhibits a lower Reynolds number and lower specificheat release).

In conventional, i.e., non-circulating, fluidized bed reactors forcombusting particulate material, the material to be combusted is fedover a bed of granular material, usually sand. In such reactors, it isdesirable to be able to vary the amount of particulate material fed tothe reactor and, concomitantly, the amount of pressurized air suppliedto the reactor over as wide a range as possible. The hydrodynamicturndown ratio of a reactor, which is defined as the ratio ofpressurized air flow at maximum reactor load to pressurized air flow atminimum reactor load, is a measure of the ability of a reactor tooperate over the extremes of its load ranges. Notwithstanding the needfor a fluidized bed reactor with turndown ratios in excess of 2 to 1, soas to improve the ability of the reactor to respond to varying powerdemands, the prior art has not satisfactorily provided a solution.

By way of example, prior art non-circulating fluidized bed boilers areknown which employ an oxidizing fluidized bed for heat generation. Insuch boilers, relatively high heat releases and heat transfer directlyfrom the fluidized bed material to heat exchange surfaces immersedtherein serve to enhance the efficiency of the boiler, thereby reducingthe boiler dimensions required to produce the desired thermal output,when compared with traditional boiler designs. Although high heatexchange efficiency is inherent in the operation of such oxidizingfluidized bed boilers, such boilers have a low turndown ratio, requiringa relatively narrow range in the variation of fuel consumption and heatoutput. These disadvantages have impeded widespread commercialization ofsuch oxidizing fluidized bed boilers.

SUMMARY OF THE INVENTION

The present invention, in a radical departure from the conventional fast(circulating) fluidized bed reactors discussed above, has overcome theabove-enumerated problems and disadvantages of the prior art bysupplying pressurized secondary air tangentially into the upper region(vapor space) of a circulating fluidized bed reactor so as to create acyclone of high turbulence, whereby the reaction rate is significantlyincreased. As used herein, and in the accompanying claims, the term"vapor space" means the region of a circulating fluidized bed combustorwhere combustion of vapor occurs, accompanied by combustion ofpreviously uncombusted solid carbon. This region is also known in theart as the "free board" region.

It is an object of the invention to provide a circulating fluidized bedreactor utilizing a cyclone of turbulent gases in a cylindrically shapedupper region of the reactor so as to provide a more intense reaction,and therefore a significantly improved reaction rate, a lower reactiontemperature (if required), and a higher specific heat release, comparedto prior art circulating fluidized bed reactors. A further object is toprovide a reactor having a shorter fluidizing gas residence timerequired to complete the reaction to the desired level. In particular,specific heat releases in excess of about 1.5 million Kcal per cubicmeter per hour are believed to be obtainable in fluidized bed combustionaccording to the present invention. The foregoing advantages will permita significant reduction in the size and, a fortiori, the cost ofconstructing the circulating fluidized bed reactor of the presentinvention. This will be true in adiabatic combustor, boiler, andgasification applications of the invention. It is anticipated, forexample, that several times less internal volume will be required for acombustor constructed in accordance with the present invention, and forboiler applications, at least about 3-5 times less heat transfer surfacearea will be needed.

A further reduction in cost is provided in one embodiment of theinvention by eliminating the need for an external solids separator(cyclone).

A further object of the invention is to provide a combustion system forburning combustible materials having a high moisture content, and a wideparticle size distribution, e.g., ranging from a few microns to tens ofmillimeters (effective diameter).

Still another object of the invention is to provide an improved boilersystem having a high turndown ratio and easier start-up than prior artsystems. It is an additional object of the invention in this regard toprovide a separate fluidized bed heat exchanger adjacent to thecirculating fluidized bed reactor for cooling the entrained solidsexiting from the reactor prior to their re-entry into the reactor. Theheat exchanger is fluidized in the bubbling regime and contains boiler,superheater, evaporator, and/or economizer coils immersed in thebubbling fluidized bed, with the further objective of significantlyreducing the heat exchanger surface area required for effective heattransfer. In such an overall system (circulating fluidized bed reactorand adjacent bubbling fluidized bed heat exchanger), it is a furtherobjective to eliminate the vertical heat exchanger tube-lined wallspreviously utilized in the upper region (vapor space) of prior artcirculating fluidized bed reactors, thereby considerably reducing thecost of constructing such a system.

To achieve the objects and in accordance with the purposes of theinvention, as embodied and broadly described herein, a method ofoperating a fast fluidized bed reactor according to the inventioncomprises: (1) providing a substantially upright fluidized bed reactorcontaining a bed of granular material and having an upper and a lowerregion, the upper region having a cylindrically shaped interior surface;(2) feeding matter to be reacted into the lower region of the reactor;(3) supplying a first stream of pressurized air to the reactor through aplurality of openings in the lower region at a sufficient velocity tofluidize the granular material in the circulating regime, whereby atleast a portion of the granular material is continually entrained upwardinto the upper region; (4) tangentially supplying a second stream ofpressurized air to the upper region of the reactor through at least oneopening in the cylindrical interior surface of the upper region(preferably two, or more, oppositely disposed openings are provided);(5) maintaining a Swirl number of at least about 0.6 and a Reynoldsnumber (related to the combustor exit gas velocity and throat diameter,D) of at least about 18,000 in the upper region of the reactor forproviding a cyclone of turbulence in the upper region which increasesthe rate of reaction in the reactor, wherein, at maximum operatingcapacity for the reactor, the second stream of air constitutes in excessof about 50% of the total pressurized air fed to the reactor; and (6)removing a portion of the granular material and reaction gases from theupper region of the reactor through an exit port situated adjacent tothe upper boundary of the cyclone of turbulence, separating the portionof the granular material from the reaction gases and returning theseparated granular material to the lower region of the reactor.

In one embodiment of the invention, the separating step is carried outin an adjacent cyclone separator. However, in accordance with anotherembodiment of the invention, cyclone separators are not utilized. Suchan embodiment is generally similar to the above-described method, exceptfor the following steps: (1) providing a closed annular chamberconcentrically surrounding at least the upper portion of the upperregion of the reactor and operatively connected at its lower end to thelower region of the reactor, the cylindrical interior surface of theupper region of the reactor having an annular gap located in its upperportion and extending into the annular chamber; and (2) passing at leasta portion of the turbulently flowing granular material from the upperregion of the reactor through the gap and into the annular chamber bycentrifugal force, thereby separating the portion of the granularmaterial from the reaction gases present in the upper region, andreturning the separated granular material by the force of gravitythrough the lower end of the annular chamber into the lower region ofthe reactor.

The present invention is directed to an improvement in a method ofoperating an upright circulating fluidized bed reactor containing a bedof granular material fluidized by a first stream of pressurized air,comprising: (1) entraining at least a portion of the granular materialin the first stream of air, thereby elevating it into a cylindricallyshaped upper region of the reactor; (2) creating a cyclone of turbulentgases in the upper region of the reactor having a Swirl number of atleast about 0.6 and a Reynolds number of at least about 18,000 forturbulently flowing the elevated portion of granular material, bytangentially introducing a second stream of pressurized air into theupper region of the reactor, wherein, at maximum operating capacity forthe reactor, the second stream of air constitutes in excess of about 50%of the total pressurized air fed to the reactor; and (3) returning theelevated portion of granular material from the upper region of thereactor to the bed of granular material at a location beneath thecyclone of turbulent gases.

Typically, the method of the present invention is performed in anadiabatic mode, in which the total pressurized air supplied is in excessof the stoichiometric amount needed for combustion or below thestoichiometric amount, i.e., for gasification conditions; or in anon-adiabatic mode in which a heat exchange surface is provided in thefluidized bed for removing heat from the bed.

In addition to the above-described methods, the present invention isalso directed to a fast fluidized bed reactor, comprising: (1) asubstantially upright fluidized bed reaction chamber for containing abed of granular material, the chamber having an upper and a lowerregion, the upper region having a cylindrically shaped interior surface;(2) means for feeding matter to be reacted into the lower region of thereaction chamber; (3) means for supplying a first stream of pressurizedair to the reaction chamber through a plurality of openings in the lowerregion at a sufficient velocity to fluidize the granular material in thecirculating regime, whereby at least a portion of the granular materialis continually entrained upward into the upper region; (4) means fortrangentially supplying a second stream of pressurized air to the upperregion of the reaction chamber through at least one opening in thecylindrical interior surface, and preferably at least two oppositelydisposed openings, the second stream being supplied, and said reactorbeing constructed, in a manner adapted to provide a Swirl number of atleast about 0.6 and a Reynolds number of at least about 18,000 in theupper region, thereby creating a cyclone of turbulence in the upperregion which increases the rate of reaction in the chamber, wherein atmaximum operating capacity for the reactor, the second stream of airconstitutes in excess of about 50% of the total pressurized air fed tothe reaction chamber; and (5) means for separating the granular materialand the reaction gases exiting from the reaction chamber through an exitport situated adjacent to the upper boundary of the cyclone ofturbulence, and returning the separated granular material to the lowerregion of the reaction chamber.

As embodied herein, the means for separating the granular material andreaction gases may comprise a cyclone separator. However, as broadlyembodied herein, the present invention is further directed to a fastfluidized bed reactor which does not include cyclone separators. Such areactor is generally similar to that described above, except for thefollowing structure: a closed annular chamber concentrically surroundingat least the upper portion of the upper region of the reaction chamberand operatively connected at its lower end to the lower region of thereaction chamber, the cylindrical interior surface of the upper regionof the chamber having an annular gap located in its upper portion andextending into the annular chamber, whereby at least a portion of theturbulently flowing granular material exits from the upper region of thereaction chamber, thereby separating the granular material from thereaction gases present in the upper region, the separated granularmaterial being withdrawn from the upper region by centrifugal force andreturned by the force of gravity through the lower end of the annularchamber into the lower region of the reaction chamber.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic vertical section view of a fast fluidized bedreactor constructed in accordance with the present invention;

FIG. 2a is a diagrammatic vertical section view of a fast fluidized bedboiler constructed in accordance with the invention;

FIG. 2b is a schematic illustration of a fluidizing air valvingarrangement suitable for use in the modified sluice shown in FIG. 2a andFIG. 4.

FIG. 3 is a diagrammatic vertical section view of a fast fluidized bedreactor constructed in accordance with another embodiment of theinvention having no cyclone separator;

FIG. 4 is a diagrammatic vertical section view of a fast fluidized bedboiler according to a further embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the presently preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

One preferred embodiment of the fast (circulating) fluidized bed reactorof the present invention is shown in FIG. 1. As shown, the reactor ofthe present invention may comprise, for example, a combustor,represented generally by the numeral 1. In accordance with thisembodiment of the invention, the combustor 1 includes a substantiallyupright fluidized bed combustor chamber 10 containing a fluidized bed ofgranular material in its lower region 11. Preferably, the interiorsurface of lower region 11 is substantially conically shaped and thecross-sectional area of the bottom of lower region 11 is smaller thanthat of upper region 18, as shown. As will be discussed more fullybelow, such a size and shape facilitates the obtaining of the requiredSwirl number, by permitting a reduction in the fluidizing gas flow and,consequently, an increase in the "secondary" air. The granular bedmaterial is preferably ash or sand, or another inert material.

The granular material is fluidized in the fast (circulating)fluidization regime with pressurized oxygen-containing gas (e.g., air),referred to herein as "primary" air, which is supplied as a streamthrough a plurality of openings 12 extending through support surface 13.As will be more fully discussed below, the primary air supplied throughopenings 12 preferably constitutes less than about 50% of the total airsupplied to combustor chamber 10, i.e., the air required for thecombustion process. Openings 12 may comprise conventional pressurizedair distribution apertures or nozzles. A source of pressurized air,e.g., blower 14, feeds the air to a plenum chamber 15 beneath supportsurface 13. Chamber 15 supplies the air to openings 12. A separateconduit 16 extends through support surface 13 for removing refuse, suchas tramp material and/or agglomerated ash, etc., from combustor chamber10.

Combustor 1 further includes means for feeding combustible matter to thelower region 11 of combustor chamber 10 through inlet 17. As embodiedherein, such means may comprise any suitable conventional mechanicalfeeding mechanism or, as shown, pneumatic feeder 20. The combustiblematter may be introduced into or above the fast fluidized bed, andundergoes complete drying, volatilization, decrepitation and partialcombustion processes in the lower region 11 of combustor chamber 10 toan extent limited by the free oxygen available in the fluidizing gas. Aportion of the granular bed material, unburnt fuel, gaseous volatilematter, solid carbon and ash is carried (i.e. entrained) by the fluegases into an upper region 18 of combustion chamber 10.

In sharp contrast to prior art fast fluidized bed reactors, the fastfluidized bed reactor in accordance with the present invention does notprovide for feeding the entrained granular bed material, unburnt fuel,solid carbon, ash, gases, etc. directly into a solids-gas separator(e.g., a cyclone separator). Rather, as noted above, the entrainedsolids and gases are carried upward into the upper region 18 ofcombustor chamber 10, where further combustion takes place.

It is generally known that the quantity of particles transported by anascending gas is a function of the gas flow velocity to the third tofourth power. Thus, greater solids reaction surface can be achieved by:(a) maintaining maximum solids' saturation in the ascending gas flow,which may be achieved by solids' return to the fluidized bed using anouter separation cyclone, and (b) increasing the vertical velocity ofthe fluidizing gas to a desired level sufficient to provide the desiredcarry-over from the fluidized bed into upper region 18. For any fuelhaving a given specific ash particle size distribution, this verticalgas velocity must be sufficiently high, as noted above, but must not beso high as to cause intensive erosion of the refractory liner, which ispreferably provided on the interior surface of upper region 18, due tovery high ash concentration in this region, as will be discussed below.The interior surface of upper region 18 is cylindrically shaped in orderto achieve swirling flow in the upper region, as discussed more fullybelow.

In accordance with the invention, means are provided for tangentiallysupplying a second stream of pressurized air (referred to herein as"secondary" air) to the upper region 18 of combustor chamber 10 throughat least one opening 19, and preferably at least two oppositely disposedopenings 19. Still more preferably, a plurality of pairs of openings 19are provided at several aggregate points in upper region 18. As shown inFIG. 1, in one advantageous embodiment the plurality of pairs ofoppositely disposed openings are vertically aligned and spaced apartthroughout upper region 18. (The cross-sectional view shown in FIG. 1necessarily depicts only one opening of each pair of openings.)

As embodied herein, a source of pressurized air, e.g., conventionalblower 14, feeds the secondary air to, for example, a vertical manifold21. As will be discussed in greater detail in the ensuing paragraphs,the secondary air preferably constitutes more than about 50% of thetotal air fed to combustor 1, i.e., the total air flow required for thereaction process. As will be brought out below, under certain limitedcircumstances, as, for example, when the temperature of the secondaryair is above ambient, the secondary air may comprise somewhat less than50%, e.g., about 30-40%, of the total air supplied.

Furthermore, it is critical that the secondary air be supplied at asufficient velocity, and that the geometric characteristics of theinterior surface of upper region 18 be adapted, to provide a Swirlnumber (S) of at least about 0.6 and a Reynolds number (Re) of at leastabout 18,000, which are required to create a cyclone of turbulence inupper region 18. Preferably, the reactor of the present invention isconstructed and operated in a manner adapted to yield these minimumvalues of Swirl number and Reynolds number when operating at minimumcapacity (i.e., on the order of 20% of maximum capacity), so that highervalues can be obtained at maximum capacity. On the other hand, the Swirlnumber and Reynolds number must not exceed those values which wouldresult in an unacceptable pressure drop through combustion chamber 10.

It is this cyclone of turbulence which enables the reactor of thepresent invention to achieve specific heat release values higher than1.5 million Kcal per cubic meter per hour when utilized as a combustor,thereby significantly increasing the rate of combustion. As a result,the size of the combustor of the present invention can be significantlyreduced, compared to prior art combustors which have a specific heatrelease of only about 0.2 million Kcal per cubic meter per hour.

The interior of the upper region 18 of combustion chamber 10 mustexhibit certain geometric characteristics, together with the applicablegas velocities, in order to provide the above-noted requisite Swirlnumber and Reynolds number. These features are discussed generally in"Combustion in Swirling Flows: A Review," supra, and the referencesnoted therein, which publications are hereby specifically incorporatedherein by reference.

By way of illustrative hypothetical example, for an adiabatic combustorhaving a capacity (Q_(com)) of 10 million Kcal/hr, a combustiontemperature (T_(com)) of 1273° K., a secondary (tangential) airtemperature (T_(air)) of 313° K. (ambient), a specific heat release (q)of 2 million Kcal/hr, and a fluidized bed bottom gas velocity (W_(FB))of 2.3 m/sec, and assuming combustion of wet wood chips having a fuelmoisture content of 55%, it can be shown that a Swirl number (S) inexcess of 0.6, a Reynolds number (Re) in excess of 18,000, and anacceptable total pressure drop across the combustor 10 can be obtainedif the combustor is properly designed and a large enough fraction (η) ofthe total air flow into the combustor 10 is introduced tangentially intoupper region 18, i.e., as secondary air. Specifically, with reference toFIG. 1, it can be shown that: ##EQU1##

It can thus be shown that, for a fuel for which ρ equals about 0.8 andwhich is combusted in such an adiabatic combustor 10 constructed andoperated such that f=2.2, Z=2.2, D_(o) =1.425m=D_(e), Y=0.1163m²/1.594m² =0.073, and the inlet and outlet aerodynamic coefficients are 2and 4, respectively, τ will then equal 0.14 sec, η will equal 0.95, Swill be 1.86, Re will be 187,724, and the total pressure drop throughthe combustor will be on the order of about 400 mm w.c., when the unitis operated at 100% capacity. When such a combustor is operated at 20%capacity, τ will be increased to a value of 0.71 sec, η will become0.765, S=1.2, Re=37,630 and the total pressure drop will be about 16 mmw.c., provided the value of Y is kept constant and the inlet and outletaerodynamic coefficients are 2 and 4, respectively.

From the above analysis, and particularly Equation No. 1, it can be seenthat a reduction in the combustion temperature will facilitate theobtaining of the requisite Swirl number. This fact may be used toadvantage in combusting wet biomass materials in accordance with thepresent invention at temperatures within the range of from about 500° C.to 1000° C., contrary to prior art teachings concerning the need forcombustion temperatures on the order of about 1000° C.

As is also apparent from the Equations set forth above, construction ofcombustor 10 in a manner such that the cross-sectional area of thefluidized bed bottom is smaller than that of upper region 18 ispreferred, since this will facilitate the obtaining of the requisiteSwirl number. This is especially important when high moisture contentfuel is used and when low pressure drops are desired. Moreover, the useof a smaller bottom cross-sectional area permits the use of higherbottom gas velocities, which, in turn, permits combustion of a fuelhaving larger particle sizes, while insuring that such particles can befluidized in the bed.

In constructing a combustor in accordance with the present invention, itis clear from the above analysis that many parameters may be varied inorder to achieve the requisite Swirl number and Reynolds number. Forexample, the values of the parameters X, Y, and Z can generally beadjusted as necessary, within the constraints imposed by the need toobtain an acceptably low pressure drop through the entire system. Inthis regard, it should be noted that the maximum acceptable pressuredrop through a combustor is generally on the order of 500-1000 mm w.c.for wet biomass combustion, and somewhat higher for coal combustion.However, as a result of the improved heat transfer exhibited by theoverall system of the present invention, a pressure drop of 1000 mm w.c.should also be achievable for coal combustion. As a further hypotheticalexample, for a non-adiabatic combustor having a capacity (Q_(com)) of7.91 million Kcal/hr, a combustion temperature (T_(com)) of 1123° K., anelevated tangential air temperature (T_(air)) of 573° K., a specificheat release (q) of 2.5 million Kcal/m³ /hr, and a fluidized bed bottomgas velocity (W_(FB)) of 2.3 m/sec, and assuming combustion of coalhaving a relatively low moisture content, it can be shown that a Swirlnumber (S) in excess of 0.6, a Reynolds number (Re) in excess of 18,000,and an acceptable total pressure drop across the combustor 10 can beobtained if the combustor is properly designed and a large enoughfraction (η) of the total air flow into the combustor 10 is introducedtangentially into upper region 18. Specifically, it can be shown, basedon the equations discussed above, that for a fuel for which ρ=0.94 andwhich is combusted in such a non-adiabatic combustor 10 constructed andoperated such that the inlet and outlet aerodynamic coefficients are 2and 4, respectively, f=1.8, Z=3.3, D_(o) =1.069 mm=D_(e), and Y=0.067 m²/0.897 m² =0.075, τ will equal 0.308 sec, η will equal 0.89, S willequal 4.75, Re will equal 90,000, and the total pressure drop throughthe combustor will be about 350 mm w.c., when the unit is operated at100% capacity. When such a combustor is operated at 20% capacity, τ willbe 1.54 sec, η will become 0.445, S=1.2, Re=18,000, and the totalpressure drop through the combustor will be approximately 15 mm w.c.,provided the value of Y is kept constant.

From the above hypothetical comparative analysis, certain conclusionscan be reached:

(1) The fraction (η) of the total air flow into the combustor which mustbe introduced tangentially as secondary air (via ports 19) in order toachieve the requisite Swirl number and Reynolds number can be reduced ifthe temperature of the secondary air (T_(air)) is increased. Similarly,η is also reduced for fuels for which the value of ρ is larger (e.g.,for lower moisture content fuels). Specifically, from the aboveequations it can be shown that, under certain conditions, e.g., T_(air)=in excess of about 150° C. and ρ=0.94, only about 30% to 50% of thetotal air need be supplied as secondary air in order to achieve a Swirlnumber in excess of 0.6, when operating the combustor described above at20% (i.e., at very low) capacity; although a value of η in excess ofabout 0.5 is still required at maximum (at or about 100%) capacity.Furthermore, such a combustor will, as shown above, exhibit a lowertotal pressure drop.

(2) Where the temperature (T_(air)) of the tangential air is nearambient, i.e., after passing through blower 14 (e.g., 40° C.), thetangential air must comprise in excess of about 50%, and preferably inexcess of about 80%, of the total air flow into the combustor at 100%combustor capacity, and must comprise in excess of about 50% of thetotal air flow into the combustor, at the lowest partial combustorcapacity desired (e.g., 20%).

(3) Items (1) and (2) above relate to the combustion of carbonaceousfuel in air. Slight variations can be expected for the reaction ofmaterials other than those mentioned above in air or other gases.However, the criticality of maintaining a Swirl number in excess ofabout 0.6 and a Reynolds number in excess of about 18,000 will notchange.

Fuel combustion is substantially completed in the cyclone of turbulencein upper region 18 at a temperature below the fusion point, whichprovides a friable ash condition.

In accordance with one embodiment of the invention, as shown in FIG. 1,combustor 1 further comprises means for separating the granular bedmaterial from the combustion gases exiting from upper region 18 throughexit port 22 located near the top of combustor chamber 10, and adjacentto the upper boundary of the cyclone of turbulence, and returning theseparated material to the lower region 11 of combustor chamber 10 viainlet port 23. As embodied herein, the means for separating the granularbed material from the combustion gases includes a suitable conventionalcyclone separator 24 (or a plurality thereof) operatively connectedbetween inlet port 23 and exit port 22 at the top of combustor chamber10. Flue gases exit from cyclone separator 24 through port 35, and arethen typically fed to the process heat supply or boiler, as the case maybe. For example, the exhaust gases exiting from cyclone separator 24 maybe fed to kilns, veneer dryers, etc.

Preferably, the separated granular material is not fed directly fromcyclone separator 24 to inlet port 23, but, instead, enters a sluice 25operatively connected between separator 24 and port 23. Sluice 25includes a standard, i.e., bubbling, fluidized bed comprised of theseparated material. The separated material is fluidized with pressurizedair supplied through a plurality of openings 26, and over-flows throughinlet port 23 into the fluidized bed in lower region 11 of combustorchamber 10. Ash tramp material may be removed through conduit 27 asneeded. Sluice 25 contains a solid partition 28 for eliminating crossflow of gases between the lower region 11, combustor chamber 10 andcyclone separator 24. Since the fluidized bed acts as a liquid, sluice25 operates in the same manner as a conventional liquid trap, andfunctions primarily to prevent the primary and secondary air supplied toreactor chamber 10 from bypassing upper region 18 of reactor chamber 10.

The present invention can be applied to most non-uniform combustibleparticulate solid materials, such as, for example, wood wastes,municipal refuse, carbonaceous matter (e.g., coal) and the like.However, it also can be used for liquid and gaseous fuel.

Additional beneficial features of the above-described embodiment of theinvention include the following: (a) low temperature combustion can beutilized, if desired, as for example, in the combustion of biomass fuelsat temperatures on the order of 500°-1000° C.; (b) the pressure drop ofseparation cyclone 24 is typically in the approximate range of 3"-6"w.c. and, if it is overcome by a draft fan (not shown), the pressure incombustor chamber 10 where fuel is fed in can be maintained at about oneatmosphere, negative or positive (this will simplify the fuel feedingsystem); and (c) due to the fact that the fluidized bed in combustor 1operates at the pneumatic transport gas velocity, which is tens of timeshigher than the terminal fluidizing velocity, the reduction of thecombustor's capacity is practically unlimited, i.e., it lies beyond 5:1.

The method of the present invention can also be used for boilerapplications which, from an economical standpoint, require low excessair for combustion and, therefore, heat absorption in the fluidized bed(lower region 11). In such embodiments, the cross section of lowerregion 11 is preferably of quadrangular shape and of a larger size inorder to accommodate a heat exchange surface of reasonable size in thefluidized bed volume. This is particularly so when the combustiontemperature and/or the fuel moisture content are low. As shown in thedashed lines in FIG. 1, the heat exchange surface may, for example,comprise a heat exchanger tube arrangement 29 in lower region 11. Thetube arrangement may be of any suitable size, shape and alignment(including vertical tubes), as is well known in the art. Preferably,heat exchanger tube arrangement 29 will be operatively connected to aprocess heat supply or to a conventional boiler drum, not shown, forboiler applications. The heat exchanger cooling media may comprise anysuitable conventional liquid or gaseous media, such as, for example,air. In boiler applications, the exhaust gases exiting, from cycloneseparator 24 are preferably fed to the boiler convective tube bank in aconventional manner.

The present invention, as broadly embodied herein, is also directed to afast fluidized bed reactor having a cyclone of turbulence in the upperregion of a reactor chamber, as described above, but wherein the needfor an external cyclone separator is eliminated, thereby permitting asignificant reduction in the size, and thus the cost, of the overallsystem.

Specifically, in accordance with the embodiment of the inventionillustrated in FIG. 3, fast fluidized bed reactor 2 includes asubstantially upright fluidized bed reactor chamber 10 containing afluidized bed of granular material in its lower region 11. For ease ofunderstanding, like reference numerals will be used, where appropriate,to identify features of this embodiment which are identical, orsubstantially identical, to those shown in the embodiment depicted inFIG. 1. The granular material is fluidized in the fast fluidizationregime in the same manner and under the same conditions as in FIG. 1 andtramp material is removed as disclosed.

Reactor 2 further includes a conventional feeder 20 for feedingcombustible particulate matter to the lower region 11 of reactor chamber10 through inlet 17 in the manner discussed in conjunction with FIG. 1.

As in the embodiment of FIG. 1, granular bed material, unburntcombustible matter, gaseous volatile matter, solid carbon and ash areentrained by the flue gases into an upper region 18 of reactor chamber10, where secondary air is tangentially supplied through a plurality ofopenings 19 in the cylindrically shaped interior surface of upper region18 in the same manner as in FIG. 1. As fully explained above, thesecondary air normally must constitute more than about 50% of the totalair fed to reactor 2, and must be supplied at a sufficient velocity suchthat, together with the geometric characteristics of the interiorsurface of upper region 18, a Swirl number of at least about 0.6 and aReynolds number of at least about 18,000 are provided in upper region18, thereby creating a cyclone of turbulent gas flow in which combustionis substantially completed.

In accordance with this embodiment of the invention, and in contrast tothe embodiment of FIG. 1, separation of the flue gases and the solidscarried thereby in upper region 18 does not require the use of aconventional cyclone separator. Rather, a closed annular chamber 30concentrically surrounds at least the upper portion of upper region 18and is operatively connected at its lower end to lower region 11 viafeed openings 31 which communicate with the fluidized bed. The interiorsurface of upper region 18 possesses an annular gap (clearance) 32located in its upper portion and communicating with annular chamber 30.The turbulently flowing granular material and other particulate solidscarried by the flue gases are entrained by the ascending cyclonic gasflow up to the gap 32. At this point, the entrained particles, beingsubjected to strong centrifugal forces, are thrown through the gap 32into annular chamber 30, and are thus effectively separated from theflue gases. The volume of flue gases present in annular chamber 30 willexhibit a spinning flow, albeit at a much lower rate of revolution. Thetangential velocities in this revolving volume of gases are sharplyreduced, with the increased radius of annular chamber 30. Consequently,as the particles enter annular chamber 30 and approach the outer wall ofthe chamber, where there is essentially no ascending gas flow, they willbe influenced by centrifugal force and the force of gravity, which willcause them to drop to the lower end of chamber 30 and fall through feedopenings 31 into the lower region 11 of reactor chamber 10 beneath thesurface of the bed.

At least one tangential secondary air port 19 must be positioned in theportion of the inner surface of upper region 18 which extends aboveannular gap 32, for the purpose of maintaining a spinning gas flow ingap 32. Preferably, at least one pair of oppositely disposed ports isprovided. Flue gases containing a very low solids concentration exitfrom the top of reactor chamber 10 through tangentially or centrallysituated exit port 22 in the manner discussed in conjunction with FIG.1.

As a result of the elimination of conventional external cycloneseparators, the embodiment of the invention shown in FIG. 3 will exhibitless pressure drop than the embodiment shown in FIG. 1.

As with the embodiment shown in FIG. 1, the fast fluidized bed reactorshown in FIG. 3 can be utilized for adiabatic combustor and boilerapplications, as well as for fluidized bed gasification. In the case ofboiler applications, as discussed in connection with FIG. 1, a heatexchanger tube arrangement 29 shown in dashed lines) is provided in thelower region 11 of the reactor.

Turning now to FIGS. 2a and 4, these figures illustrate furtherembodiments of the invention generally similar in structure andoperation to the embodiments shown in FIGS. 1 and 3, respectively, buthaving significantly higher turndown ratios. Like reference numeralshave been used in FIGS. 2a and 4 to identify elements identical, orsubstantially identical, to those depicted in FIGS. 1 and 3,respectively. Only those structural and operational features which serveto distinguish the embodiments shown in FIGS. 2a and 4 from those shownin FIGS. 1 and 3, respectively, will be described below.

In particular, the embodiments shown in FIGS. 2a and 4 include a coolingfluidized bed 40 (with a heat exchanger) situated immediately adjacentto the lower region 11 of reactor chamber 10 and having an overflowopening 41 communicating with lower region 11. Cooling fluidized bed 40comprises an ordinary (i.e., bubbling) fluidized bed of granularmaterial, and includes a heat exchange surface, shown here as heatexchanger tube arrangement 42, which contains water or another fluid,such as, for example, steam, compressed air, or the like. The fluidentering tube arrangement 42 is preferably supplied from a conventionalboiler steam drum (not shown). The bed is fluidized in a conventionalmanner by tertiary pressurized air supplied from a plenum 43 throughopenings 44 in a support surface, and ashes are removed (when required)through conduit 45.

The fluidized bed is comprised of the granular material and other solidsseparated in cyclone separator 24 or annular chamber 30, as the case maybe. These solids are thus at a relatively high temperature. Heatexchanger tube arrangement 42 functions as a cooling coil to cool thefluidized bed, with the cooled solids overflowing the bed throughopening 41 and re-entering lower portion 11 of reactor chamber 10 to beagain fluidized therein. The fluid passing through tube arrangement 42is consequently heated and preferably fed, for example, to aconventional boiler drum (not shown) in a steam generation process.

The embodiments illustrated in FIGS. 2a and 4 further include a modifiedfluidized bed sluice 50 which is divided into three compartments 51, 52and 53 by substantially solid partitions 54 and 55. Each of thesecompartments is fluidized in a conventional manner by a separate,regulatable stream of pressurized fluidizing air from separatefluidizing aperture systems 61, 62, and 63, respectively. (See FIG. 2balso.) Aperture systems 61, 62 and 63 are regulated by separate valves71, 72, and 73, respectively. Compartments 51 and 52, together, functionin the same manner as sluice 25 (FIG. 1), described above, to preventcross flow of gases, with the separated solids from cyclone separator 24(FIG. 2a) or from annular separation chamber 30 (FIG. 4) enteringcompartment 52 and being overflowed from compartment 51 through opening23 into the fluidized bed in lower region 11 of reactor chamber 10.However, as will be described below, when the fluidized bed reactor 10is in normal operation (functioning at full or partial loads),compartment 51 is not fluidized, and therefore plays no role in therecirculation of solids.

In normal operation, compartments 52 and 53, but not 51, are fluidized,i.e., valves 72 and 73 are open and valve 71 is closed. As a result,separated solids enter compartment 52 and overflow into coolingfluidized bed 40 as shown.

For a better understanding of how this embodiment functions to improvethe turndown ratio, the required procedure to initially place it intooperation from the cold condition to a full load and then turn it downto a desired level will be explained.

The ignition burner (not shown), preferably located above the lowerregion 11, is turned on, while primary, secondary, tertiary and sluiceair are shut off. At the time when the combustor's refractory and itsinternal volume temperature exceed the solid fuel ignition temperature,the primary air, secondary air and sluice air are partially turned on,while compartment 53 of sluice 50 remains shut off (valve 73 is closed,FIG. 2b). From this moment, an adiabatic fluidized bed combustor schemeis in operation in reactor chamber 10, and when the temperature againexceeds the solid fuel ignition temperature, solid fuel is fed into areactor chamber 10. After the solid fuel is ignited and, consequently,the exit gas temperature has risen, additional sluice air is thensupplied to compartment 53 (by opening valve 73), and a fraction of thetertiary air is supplied. To keep the combustion temperature on therise, at this time the secondary air flow is gradually increased, with asimultaneous increase in the solid fuel feed rate, and the ignitionburner is shut off. If the gas exit temperature continues to rise, afurther increase of secondary air flow and fuel feed rate should bepursued. At the point when the gas exit temperature achieves its highestdesigned level, the tertiary air flow rate must be continuouslyincreased until it reaches its full rate. Simultaneously, the fuel rateand secondary air rate are also continuously increased. To achieve fullload, the sluice compartment 51 air flow valve 71 (FIG. 2b) is closeduntil it is completely shut off. At this moment, if the gas exittemperature is at the desired level, the secondary air flow and fuelrate are not increased any further, and are then maintained inaccordance with the fuel-air ratio required to obtain the mosteconomical fuel combustion. At this point, the reactor can be consideredas having full load (100% capacity). The minimum capacity of thereactor, i.e., desired turndown ratio, can be obtained if the sequenceof operations outlined above is followed in reverse order, until thepoint where the ignition burner is shut off. By changing the sluice airflow in compartment 51 (by fully or partially closing valve 1,corresponding to the desired combustion temperature) and by changing thetertiary air flow, the combustion temperature can be further controlledat any desired combustor capacity (including maximum capacity, providedthe surface of heat exchanger 42 has been over designed, i.e., so as tohandle more than the amount of heat transfer normally contemplated).

In brief review, the key feature, in terms of obtaining a high turndownratio according to the embodiments depicted in FIGS. 2a and 4, is thefact that the cooling fluidized bed heat exchange surface 42 may begradually pulled out (but not physically) from the combustion process soas to keep the fuel-air ratio and combustion temperature at the requiredlevels. Further, in addition, due to the fact that the fluidized bed ofthe combustor chamber operates at the pneumatic transport gas velocity(recirculation of most of its inventory) and is fluidized by air flow ofmuch less than 50% (generally less than 20%) of the total air flow, theturndown ratio, from a hydrodynamic standpoint, is practicallyunlimited, i.e., lies beyond 5:1.

Furthermore, the above-desired boiler turndown ratio improvement has anadditional advantage over known circulating fluidized bed boilers.Specifically, it requires less than one-half the heat exchange surfaceto absorb excessive heat from the circulating fluidized bed, due to thefollowing: (a) the tubular surface 42 fully immersed in fluidized bed 40is fully exposed to the heat exchange process, versus the verticaltube-lined walls in the upper region of the combustion chamber of priorart circulating fluidized bed boilers, in which only 50% of the tubesurface is used in the heat exchange process; (b) the fluidized bed heatexchange coefficient in such a system is higher than that for gases,even heavily loaded with dust, and vertical tube-lined walls confiningthe combustion chamber of prior art circulating fluidized bed boilers.The latter fact results, in part, from the fact that it is possible, byusing a separate fluidized bed 40, to utilize the optimum fluidizationvelocity therein, and the fact that fluidized bed 40 is comprised ofsmall particles, e.g., fly ash.

If low temperature combustion is needed, it can be utilized inconjunction with the above-described boiler turndown ratio improvement,with the consequent effect upon aggregate combustor performance asdescribed above.

It will be apparent to those of ordinary skill in the art that variousmodifications and variations can be made to the above-describedembodiments of the invention without departing from the scope of theappended claims and their equivalents. As an example, although theinvention has been described in the environment of combustingparticulate material, such as wood wastes, municipal refuse,carbonaceous material, etc., it is apparent that the apparatus andmethod of the invention can be used in other environments in whichfluidized bed reactors find utility, such as, for example, gasificationand various chemical and metallurgical processes.

What is claimed is:
 1. A fluidized bed reactor, comprising:(a) asubstantially upright fluidized bed reaction chamber for containing abed of granular material, said chamber having an upper region and alower region, said upper region having a cylindrically shaped interiorsurface; (b) means for feeding matter to be reacted into said lowerregion of the reaction chamber; (c) means for supplying a first streamof pressurized air to said reaction chamber through a plurality ofopenings in said lower region at a sufficient velocity to fluidize saidgranular material in the circulating regime, whereby at least a portionof the granular material is continually entrained upward into said upperregion; (d) means for tangentially supplying a second stream ofpressurized air to said upper region of the reaction chamber through atleast one opening in said cylindrically shaped interior surface, saidsecond stream being supplied, and said reactor being constructed, in amanner adapted to provide a Swirl number of at least about 0.6 and aReynolds number of at least about 18,000 in said upper region, therebycreating a cyclone of turbulence in the upper region which increases therate of reaction in said chamber, wherein, at maximum operating capacityfor the reactor, the second stream of air constitutes in excess of about50% of the total pressurized air fed to the reaction chamber; (e) meansfor separating the granular material and the reaction gases exiting fromsaid reaction chamber at a location adjacent to the upper boundary ofsaid cyclone of turbulence, and returning the separated granularmaterial to said lower region of the reaction chamber; (f) a secondfluidized bed fluidized in the bubbling regime and situtated in saidlower region of said reaction chamber adjacent a partition formed insaid lower region for partitioning said circulating fluidized bed fromsaid second fluidized bed, said second fluidized bed receiving andfluidizing at least a portion of the granular material separated fromthe reaction gases, said partition being situated so as to permit aportion of the granular material from said second fluidized bed tooverflow said partition into said circulating bed; and (g) first heatexchange surface means immersed in said second fluidized bed forremoving heat therefrom.
 2. A fast fluidized bed reactor as claimed inclaim 1, wherein said means for separating the granular material andreaction gases comprises a cyclone separator operatively connectedbetween said exit port and an inlet port located in said lower region ofthe reaction chamber, whereby the granular material and reaction gasesenter the cyclone separator via said exit port and the separatedgranular material returns to said lower region via said inlet port.
 3. Afluidized bed reactor as claimed in claim 1, further comprising secondheat exchange surface means situtated in said circulating fluidized bedin the lower region of the reaction chamber for removing heat from thecirculating fluidized bed.
 4. A fluidized bed reactor as claimed inclaim 3, further comprising boiler means operatively connected to saidfirst and second heat exchange surface means.
 5. A fluidized bed reactoras claimed in claim 1, wherein the horizontal cross-sectional area ofsaid upper region of said chamber is larger than that of the circulatingfluidized bed in said lower region of said chamber.
 6. A fluidized bedreactor as claimed in claim 5, wherein the portion of said lower regionof said chamber surrounding the circulating fluidized bed is conicallyshaped.
 7. A fast fluidized bed reactor as claimed in claim 1, whereinsaid means for tangentially supplying a second stream of pressurized airto said reaction chamber includes a plurality of openings in saidcylindrical interior surface.
 8. A fast fluidized bed reactor as claimedin claim 7, wherein said means for tangentially supplying a secondstream of pressurized air to said reaction chamber includes a pluralityof pairs of oppositely disposed openings in said cylindrical interiorsurface.
 9. A fluidized bed reactor as claimed in claim 8, wherein saidplurality of pairs of oppositely disposed openings are verticallyaligned and spaced apart throughout said upper region of the reactionchamber.
 10. A fluidized bed reactor as claimed in claim 1, furthercomprising boiler means operatively connected to said first heatexchange surface means.
 11. A fluidized bed reactor as claimed in claim1, wherein at least the portion of said lower region of said chambersurrounding said second fluidized bed is quadrangularly shaped in crosssection, when viewed from above.